Network diagnostic meter

ABSTRACT

A portable test meter is disclosed for measuring, calculating, recording and monitoring significant system parameters for a local communication network of the type used in industrial automation applications. The meter is disclosed in the context of a network employing the DeviceNet protocol. For each network parameter of interest, a rotary switch selects a desired one for display. An auto search mode selectable by the operator, increments through selected network parameters and compares measured or calculated values against stored data representing acceptable, marginal, or unacceptable conditions, and an associated visual indicia is displayed for each unacceptable or margin condition detected.

RELATED APPLICATION

[0001] This application is a non-provisional application claiming thebenefit of the filing date of copending U.S. Provisional ApplicationsNo. 60/248,046 filed Nov. 13, 2000 and No. 60/287,528 filed Apr. 30,2001.

FIELD OF THE INVENTION

[0002] The present invention relates to industrial communicationnetworks; and more particularly, it relates to a diagnostic test systemfor performing a wide range of diagnostic tests in situ on an industrialcommunication network using a hand-held meter of convenience size.

BACKGROUND OF THE INVENTION

[0003] Industrial automation installations have used programmable logiccontrollers (PLCs) in combination with a variety of individual devices,such as presses, robots, visual displays, solenoid actuators and manyothers, for achieving efficient information communication and control inmanufacturing environments. A communication network employing its ownprotocol and signal specifications is typically used to interface all ofthe various devices required to be connected together. One networkprotocol has grown to become the current leading device-level networkstandard for industrial automation. That is the DeviceNet protocol.Thus, the present invention, though not so limited, is disclosed in thecontext of such a network.

[0004] As with any network, there is a need to test the network duringoriginal installation and, later, during normal preventive maintenanceas well as when faults occur to ensure proper operation. Some networkanalysis tools simply establish continuity and perform no diagnosticmeasurements of active devices normally operated on the system nor ofthe signal levels or data streams typically employed in the system. Suchtools are of limited use in the complex networks of modern industrialautomation systems.

[0005] Networks of this type, considering the physical connections, thenature of the various types of devices which may be employed, includingPLCs and personal computers and display devices, as well as the signalprotocols, can be very difficult to diagnose when a problem arises.Obviously, in the case of a system which is operating a production line,if a problem arises, there is a need to identify immediately and correctthe problem. Because of the different layers of physical and electronicsystems employed, the skill level of a person adequate to performdiagnostic routines at all levels has become quite high. Yet, it is noteconomical to have a specialized person of such a high skill levelsimply waiting until a fault occurs because, despite the occurrence ofoccasional faults, the systems are quite reliable.

[0006] It is much more economical to have the initial measuring ortesting of the network conducted by maintenance personnel normally inthe manufacturing area on a regular basis, such as electricians, and toprovide such personnel with simple, easy-to-use and readilyunderstandable equipment which is economical to purchase, and may beassembled rapidly to the troubled network without the need to shut thesystem down.

SUMMARY OF THE INVENTION

[0007] The present invention addresses the problem of providing aneconomical test meter having the capability of performing test anddiagnostic measures on an industrial automation communication network insitu by personnel having the training at the level of an electrician orthe like, rather than an electronic engineer or network specialist.Depending upon the nature of the apparent problem, the on-site personnelmay then correct the problem or consult an engineer or other specialisthaving further expertise in the area. In addition, the present inventionenables the technician or operator to store (or “freeze”) data recordedduring his or her diagnostic test procedures on the network forsubsequent use or subsequent analysis.

[0008] The invention is provided in the form of a meter in a housingsized to fit comfortably in one hand of a user, leaving the other handfree to rotate a switch to the various test positions. A connector isprovided for coupling the device to the network under study. Signallines couple the data signals as well as the bus power supply voltage(“Vbus”) and the voltage of the shield lead (“Vshield”) to signalconditioning circuitry.

[0009] A display (liquid crystal display, or LCD in the illustratedembodiment) is included. The display is operated by an interface dataprocessor coupled to the main data processor (simply “processor”) bymeans of a bus. Additional memory is provided for the main processor.

[0010] As used herein, a “network” includes a series of devices, such ascomputers, programmable logic controllers, displays, sensors, controlelements or the like, commonly used in industrial automation systems. A“node” is any such device connected to the network, including, forexample, the test device of the present invention. According toDeviceNet protocol (an industrial communication protocol standard basedon the CAN or Controller Area Network, protocol standard), there are twodata signal lines, one designated as the CAN High or “CANH” line and theother is designated CAN Low or “CANL”, according to convention. Thesystem of the present invention employs a main processor receiving theincoming data signals and sampling those signals for an accuratedetermination of amplitude which is then converted to a correspondingdigital signal representation and stored in memory to establish adatabase of recorded signal measurements for various network signalparameters, to be described. The operator uses the rotary switch toaccess data stored in the main processor memory and to display thatdata.

[0011] The present system provides a wide range of diagnosticinformation, disclosed in more detail within. By way of example, twoswitch positions (positions 10 and 11 in the illustrated embodiment) maybe selected by the operator to display the CANL voltage for therecessive bit (switch position 10), and for the dominant bit (position11) of the signal. As each measurement for a given data frame is made,the system stores a “live” value, a maximum value and a minimum valuefor both the CANH and CANL signals, for the dominant bit and therecessive bit. These values in the illustrated embodiment are notassociated with a specific node, although the invention is not solimited because the data identifying the node is available to theprocessor, if desired. The “live” values are overwritten in memory sothat only the most current signals are retained in memory. However, themaximum and minimum values are replaced only if the previously storedvalues are exceeded (in absolute value).

[0012] The main processor also computes a signal value referred to asthe Relative Node Common Voltage from the recorded database and thendetermines the worst-case (i.e., maximum) difference between any twoRelative Node Common Voltages for the system under test. This differenceis defined as the network Common Mode Voltage; and it is deemed to be asignificant factor in determining whether the network is acceptable,marginal or unacceptable in its operation. The network Common ModeVoltage is also displayed upon the operator's election (switch position5).

[0013] Switch positions display the differential instantaneous voltagebetween CANH and CANL. The main processor subtracts the CANL voltagefrom the CANH voltage, and the system displays the differential voltagefor the dominant bit (rotary switch position 7) and the recessive bit(rotary switch position 6). Again, live MAX and MIN values are storedand may be displayed by the operator. The system computes and displaysthe Common Mode Voltage measured by the system in switch position 5.

[0014] These measurements have been found to provide substantialinformation to the user in isolating and diagnostic network problems. Ofparticular significance is the fact that the main processor storespredetermined limits for ranges of acceptance for each of the conditionsbeing measured and divides or groups the ranges of potentialmeasurements into “acceptable,” meaning that the parameter beingmeasured and displayed is within normal operating ranges, “marginal,”meaning that the particular parameter being measured and displayed isacceptable but not within the desired range, or “unacceptable,” meaningthat the value is outside operating specifications. Each of these threeconditions is indicated by actuating a graphic representation of thecondition which, in the illustrated embodiment, takes the form of a“happy” face indicating acceptable operating conditions, a “neutral”face indicating marginal conditions or a “sad” face indicatingunacceptable conditions.

[0015] Other positions on the rotary switch display measurements of buspower voltage, bus data traffic, bus errors and shield voltage, as willbe discussed further below.

[0016] Other features and advantages of the present invention will beapparent to persons skilled in the art from the following detaileddisclosure of the exemplary embodiment accompanied by the attacheddrawing wherein identical reference numerals will refer to like parts inthe various views.

BRIEF DESCRIPTION OF THE DRAWING

[0017]FIG. 1 is a diagram, partly in functional block form,incorporating the present invention;

[0018]FIG. 2 is an illustration of representative CANH and CANL datasignals and indicating some of the measurements taken by the instantsystem;

[0019]FIG. 3 is a diagram of data signals on the network for fourdifferent nodes, illustrating the definitions of Common Mode Voltage;

[0020]FIG. 3A is a diagram illustrating acceptable limits to the rangeof transceiver Common Mode Voltage;

[0021]FIG. 4 is a more detailed, functional block diagram of the system;

[0022]FIG. 5 is diagram illustrating an idealized voltage of thedominant and recessive bit signals on the CANH and CANL wires;

[0023] FIGS. 6-8 are more detailed schematic diagrams of the circuitry;and

[0024]FIG. 9 is a flow chart showing the operation of the interfaceprocessor in the Auto Search mode.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENT

[0025] Turning first to FIG. 1, reference numeral 10 generallydesignates a diagnostic meter for a communication network of the typeused in industrial automation applications. The meter 10 includes acasing 11, preferably made of rubber or other moldable protective,elastomeric material for both buffering any force due to a fall, in theevent the instrument is dropped, and for providing protection againstwater, oils and other liquids. On the face 12 of the housing 11, thereis a display 13. Beneath display 13 are three push buttons for actuatingassociated switches. These are referred to respectively as the“Advancing” or incrementing switch 15, the “Plus” switch 16 and the“Minus” switch 17, and they will be further described below. Beneath thedisplay 13 and the switches 15-17 is a rotary switch 19 which is itselfof conventional design and has twelve individual stations, designatedrespectively 1-11 and “Auto Search” at the twelve o'clock (top)position. Although a rotary switch is shown and preferred, personsskilled in the art will appreciate that other switch types orcombinations for selecting positions or stations may also be employed.

[0026] The meter also includes a toggle switch 20 for turning the meteron and off, and a second toggle switch 21 for placing the system eitherin a “Run” mode or a “Lock” mode, to be further described below. At thebottom of housing 11 is a threaded nipple electrical connector 23 forreceiving a cable connected to the network under test.

[0027] Turning now to the left side of FIG. 1, reference numeral 25designates a connector corresponding to the threaded connector 23 of thehousing, which may be connected to a test cable. Output lines andsignals from the network are coupled through the connector 25 to aconventional transceiver 27, the output of which feeds a main dataprocessor 28 (or simply “processor”). The main processor 28 communicateswith a second processor referred to as the “interface” processor anddesignated by reference numeral 30. Persons skilled in the art willappreciate that although two processors are shown in this embodiment,one data processor of sufficient capacity and storage could be employed.Further, if it is not necessary to display the data measured, theinterface processor may be unnecessary. The rotary switch 19 of themeter 10 feeds a signal to the interface processor 30 to indicate whatinformation stored in the system, the operator of the meter wishes todisplay. The interface processor 30 receives digital signals from thedatabase of the main processor 28, where the data is stored.

[0028] Turning now to FIG. 4, there is shown a more detailed functionalblock diagram of the system. The network connector is again identifiedby reference numeral 25, and it includes separate lines designated 52,53, 54 and 55 for the CANH voltage, CANL voltage, Vbus voltage, andVshield voltage respectively. Each of these lines is coupled toconventional signal conditioning circuitry included within the blocks56A-56D respectively. The signal conditioning circuits have their analogoutputs converted respectively by analog-to-digital converters 57A-57D.Separately, the CANH and CANL lines are also connected to the input of aconventional transceiver 59. The output of the transceiver 59, and thefour analog-to-digital converters 57A-57D (representative respectivelyof the CANH, CANL, Vbus and Vshield signals, in their associated digitalrepresentations), are coupled to the main data processor, againdesignated 28 which is coupled to the interface processor by means of aparallel bus 60. Associated with the main processor 28 is a section ofrandom access memory schematically designated at 62; and as alreadyindicated, the interface processor 30 drives the LCD display 13.

[0029] For each signal appearing on a data line, as well as for the busvoltage 54 and the shield voltage 55, the main processor controls thesample timing (described in further detail below) taken by theanalog-to-digital converter associated with each of the lines, and thatsignal is stored (copied) in the database compiled in RAM 62. For eachof the CANH and CANL signals, both dominant and recessive bits, threeseparate values are stored or copied, as described further below.

[0030] By way of explanation, FIG. 5, shows idealized voltage waveformsfor the CANH line (shown in dotted line) and the CANL line (shown insolid line). Also shown and illustrated by a “bulls-eye” symbol aretimes for each of the respective bits at which a sample is taken by theassociated analog-to-digital converter as illustrated in FIG. 1. This isthe normal CAN sampling point for DeviceNet networks, and measures theactual voltage applied to a CAN transceiver at the instant the CAN MAC(media access controller) is defining the bus state through sampling.The bus voltage is sampled at the normal CAN sampling point measuringthe actual voltage at the input of the CAN transceiver at the instantthe CAN MAC is determining the bus state. This is in contradistinctionfrom some diagnostic techniques which measure the average signal voltage(e.g., by analog filter). Network traffic affects the results in thistechnique because of the varying relationship over time of dominant torecessive bits. It has been found that by sampling the dominant andrecessive bits on the CANH and CANL lines at these sample times, anaccurate representation of the amplitude of the associated signals,particularly as perceived by the transceivers at nodes in the network,enhances the overall measurement accuracy of the network. The CANprotocol defines a bus access arbitration method that results in thepossibility of multiple nodes transmitting concurrent dominant bitsduring the start of a communication, it has been found that sampling thedominant bit on the CANH and CANL lines during the latter part of acommunication where only one node may be transmitting is desirable toavoid inaccuracies in representation of amplitude of the associatedsignals. In the case of DeviceNet, it is preferred to sample a dominantbit which occurs after the part of a communication where multiplestations may be concurrently transmitting dominant bits. This may beachieved by sampling the RTR bit in each CAN frame. Differentialvoltages (i.e., the difference between the CANH and CANL signals levels)which are greater than the amount defined by the dashed lines in FIG. 5are deemed by the transceivers as being dominant bits; and differentialvoltages which are less than this amount (i.e., falling within the rangedefined by the dashed lines) are deemed to be recessive bits.

[0031] Turning now to FIGS. 6-8, there is shown a still more detailedschematic of the system. Referring first to FIG. 6, in the upper lefthand corner, there is shown a circuit schematic diagram of the networkconnector 25, together with the leads 52 and 53 carrying the CANH andCANL signals which are also connected to their associated signalconditioning circuitry designated generally 56A and 56B in FIG. 6. Theoutputs of the signal conditioning circuitry 56A, 56B are connected toanalog-to-digital converters 57A, 57B, in FIG. 7.

[0032] The analog signal conditioning circuitry for the bus power 56C,shield voltage 56D and CANH/CANL signals are conventional and may be thesame for each signal. Returning to the connector 25, the shield lead isdesignated 54, the Vplus lead is designated 55, and the ground lead isdesignated 68. Leads 54 and 55 are connected to the input of theirassociated signal conditioning circuits 56A, 56B shown within the block56 in FIG. 6. The Vplus signal is also connected to the network powerswitch 20 and through a conventional power regulator 69 for supplyingthe various required voltage levels for system power. In the lowercentral portion of FIG. 6, there is shown a previously described on/offswitch generally designated 20 and the lock switch at 21 shown in theirmechanical counterparts in FIG. 1. These switches generate signalsdesignated respectively PWRON and LOCK which are connected to thesimilarly designated inputs respectively at pins 16 and 17 of theinterface data processor 30 in FIG. 8.

[0033] Referring now to FIG. 7, the output signals of the signalconditioning circuitry of FIG. 6 which shape the signals for Vplus 54and Vshield 55 are coupled respectively to the analog-to-digitalconverters 57C and 57D in FIG. 7, which are connected to the main dataprocessor 28.

[0034] The CANH and CANL signal lines are coupled to the data processor28 via transceiver 59, previously described in connection with FIG. 4.

[0035] Referring back to FIG. 8, the parallel (SX) communication bus 60between the main processor 28 and the interface processor 30 is shown at60 in both FIGS. 8 and 7. As mentioned, a single processor of sufficientcapacity could be used, if desired. The analog-to-digital converters57A-57D for the CANH line 52, CANL line 53, Vpower line 54 and Vshieldline 55 are also seen in FIG. 7.

[0036] The three push button switches shown at 15, 16 and 17 beneath thedisplay 13 in FIG. 1 are shown in the upper left hand portion of FIG. 8,together with their associated connections to the main processor 30. Inthe lower left hand corner portion of FIG. 8, there is shown thetwelve-position rotary switch 19 and its associated connections to themain processor 30. The outputs of the processor 30 which drive the LCDdisplay 13 are designated respectively SEG0-SEG17 and COM-0 throughCOM-3.

[0037] Referring now to FIG. 2, there are shown two waveforms designatedgenerally respectively by reference numerals 36 and 37 and representingidealized voltage waveforms appearing on the CANH and CANL wiresrespectively. The CANH and CANL signals are two of four signals coupledthrough switch 25 to the diagnostic meter. Each of the CANH and CANLsignals shown in FIG. 2 is comprised of a series of recessive bits anddominant bits, thus comprising a binary data transmission network. Thus,CANH signal 36 includes a first recessive bit 38, followed by a dominantbit 39, and then another recessive bit, a portion of which is shown at40. Similarly, the CANL signal 37 is comprised of a first recessive bit41, followed by a dominant bit 42 and then a second recessive bit 43. Asis conventional the recessive bits for both CANH and CANL voltagesignals coincide in time, as do the dominant bits.

[0038] For the DeviceNet protocol, specifications are set on all signallevels, but for purposes here, it will be observed that the differencebetween the CANH and CANL signals for a recessive bit is small. That is,the signal level of the recessive CANH bit 38 is very close to that ofthe recessive CANL bit 41. For the dominant bit, the difference betweenthe CANH signal 39 and the CANL signal 42 is, according to systemspecifications, between 1.2 and 3.0 volts. The voltages shown in FIG. 2are idealized signals appearing across a node. That is, the idealizedsignals shown in FIG. 2 are not affected by voltage drops in the linesextending between nodes or between the power source and an individualnode. As used herein, the term “Node Common Voltage” is the voltage atthe negative terminal of a node—that is, the node terminal connected tothe corresponding negative terminal of the network power supply—relativeto the voltage at the negative terminal of the network power supply, orother common point in the negative power feed line.

[0039] The recessive voltage is determined by a number of factorsincluding the number of nodes on the network and the Node Common Voltage(caused by voltage drop in the power supply feed wire) of each node. Therecessive voltage is nominally 2.5V with no Node Common Voltagevariance. With respect to an individual node's Node Common Voltage, therecessive voltage can range from −3V to 8.5V in a properly designednetwork—that is, the network will operate reliably within this range.The dominant voltage is created by a transceiver's “pulling” the CANHline toward its 5V power supply (referenced to its associated NodeCommon Voltage) and the CANL line towards its associated Node CommonVoltage.

[0040]FIG. 3 illustrates signals for CANH and CANL as viewed from acommon point in the network. It will be appreciated that if there are,by way of illustration and without limitation, twenty-four nodes (i.e.,devices such as a robotic actuator, a visual display, a programmablelogic controller, a personal computer, or the like), there will bedifferences in the terminal voltages appearing at each individual nodefrom the other nodes because of variances in Node Common Voltage at thedifferent nodes as well as voltage drops in the lines connecting thepositive terminals of the nodes with the positive terminal of the systempower supply. This normally does not interfere with proper systemoperation since the transceivers operate on voltage differences, notabsolute levels, provided the levels are within system specifications.Thus, when viewing from a single point or port in a network, thevoltages at the different nodes may differ when they are observed from aparticular point. Yet, all are acceptable signals because the absolutedifferential signals which appear across any particular node (asdistinguished from the observation point) are within systemspecifications. Thus, the transceivers for every node will still operatereliably.

[0041] Since each transceiver pulls the CANH signal towards its 5V powersupply rails which are referenced to a node's Node Common Voltage, asdefined above, and CANH and CANL lines are connected together by aterminating resistor, it is possible for both CANH and CANL voltages tomove in the same direction (i.e., positive or negative) from therecessive voltage. The voltages illustrated in FIG. 3 are the CANH andCANL signals viewed from a single point during a dominant bit for fourseparate message frames and four different nodes. CAN signals are notnecessarily symmetrical. The illustrated signals are all valid andlikely to be observed in properly operating networks, providing theCommon Mode Voltage is within prescribed described limits.

[0042] In FIG. 3, there are shown four separate dominant bits designatedrespectively B1, B2, B3 and B4. Each of the bits B1-B4 appears in adifferent message frame sequence, separated by the vertical dashedlines. Each time sequence B1-B4 is in effect extracted from a completemessage frame for an individual node, although only one dominant bit isshown in the drawing. Thus, the bit B1 is only a single dominant bitappearing out of a sequence of bits comprising a complete message frameand associated with one specific node. Similarly, the bit B2 is a singledominant bit appearing out of a sequence of bits comprising a messageframe and associated with a different node, and so on. The purpose ofthe illustration of FIG. 3 and the explanation provided is to define theterm “Common Mode Voltage” as it is used herein, and to be distinguishedfrom “Node Common Voltage”, defined above.

[0043] Each of the bits B1-B4 of FIG. 3 has a median or average voltagelevel indicated by the horizontal dashed lines associated with each ofthe illustrated bits and designated respectively RNCV1, RNCV2, RNCV3 andRNCV4. RNCV stands for the term “Relative Node Common Voltage.” It isthe voltage that is midway between the maximum voltage (CANH) and theminimum voltage (CANL) for the associated dominant bit. In other words,it is the difference between the Node Common Voltage for a specific nodeand a specific reference point in the network (typically the negativeinput terminal of the measuring device, i.e., the meter in this case).

[0044] The largest or maximum difference of the Node Common Voltagebetween any two nodes communicating on the network is the Common ModeVoltage of the network. That is, the data processor determines the NodeCommon Voltage for each node, determines the maximum and the minimumvalues and then determines the difference between the maximum andminimum values.

[0045] As discussed above and to be discussed further below, the networktransceiver circuits in each node are responsive only to thedifferential input voltage, but the input voltage levels must meetsystem specifications for proper operation. An important parameter forthe diagnostics of the present invention is the worst-case maximumCommon Mode Voltage between any two nodes on the network under test.This voltage may be calculated by subtracting the lowest measured NodeCommon Voltage from the highest measured Node Common Voltage. However inthe illustrated embodiment, it may be calculated by using Relative NodeCommon Voltage illustrated in FIG. 3 by the arrow 46 which is thedifference between the maximum Relative Node Common Voltage RNCV3 andthe minimum Relative Node Common Voltage RNCV2 for all nodes in thenetwork.

[0046] Referring now to FIG. 3A, there are shown CANH and CANL signalsfor four separate bits, again designated respectively B1-B4, and eachrepresentative of a dominant bit in a different message frame appearingat the transceivers of four different nodes. As persons skilled in theart are aware, transceivers for CAN networks are circuits which decodethe digital voltages on the network by measuring the differentialbetween the CANH signal and the CANL signal. Transceivers are designedto operate with a limited range of input signal voltages. If the inputsignal voltage exceeds the transceiver's capabilities, dominant andrecessive bits may be improperly decoded, resulting in communicationerrors. The CAN chips used in DeviceNet networks are designed to workwith data signals in the range of 0-5V, and to operate with as much as +or −5V of Common Mode Voltage. This results in an acceptable inputsignal range from −5V to +10. These levels are relative to eachrespective Node Common Voltage.

[0047] Referring then to FIG. 3A, it can be seen that bit B1 isacceptable because the CANH and CANL levels for both recessive anddominant bits fall well within the acceptable range of −5V to +10V.Turning to bit B2, it is unacceptable because the CANL level for thedominant bit, designated by reference numeral 48, falls below the lowerlimit of the acceptable range. Similarly, bit B3 is unacceptable becausethe CANH voltage for the dominant bit exceeds the uppermost limit of theacceptable range. The final bit, B4, falls entirely within the range,and therefore is acceptable.

[0048] CAN Measurements Taken

[0049] The system of the present invention makes several direct andindirect (calculated) voltage measurements. They are identified belowaccording to the time relative to each message frame during which thevoltage is measured or a computation is made, or a measurement stored.In the case of DeviceNet sampling the RTR bit in each CAN frame avoidsproblems or ambiguities that may result in the initial portion of acommunication in which multiple stations may be transmitting dominantbits concurrently.

[0050] (A) Measurements Taken During Each Frame Received

[0051] The data processor simultaneously detects and records CANH andCANL voltages “live,” maximum and minimum, during one dominant bit ofeach message frame. In the case of DeviceNet, the RTR bit is alwaysdominant (i.e., the binary bit is always a logic “zero”).

[0052] (B) Measurements Taken and Calculations Made After Each FrameReceived

[0053] The system reads the CANH and CANL dominant voltage measurementscaptured during the received frame, and it copies or writes CANH andCANL dominant measurements into the database. The previous “live” valuesare overwritten so that the “live” values in the database which will bedisplayed are the most current measurements.

[0054] The system updates the minimum and maximum CANH and CANL dominantbit measurements in the database, if appropriate. The maximum andminimum values of CANH and CANL voltages stored in memory areoverwritten if (i.e., the new measurement replaces the formermeasurement) the current value for each parameter is greater or lessthan the previous value stored in the database respectively.

[0055] The processor calculates the CANH/L dominant differentialvoltage. This is done by subtracting the CANL value from the CANH valuefor an associated bit and message. The processor copies the CANH/Ldominant differential measurement into the database. The previous “live”value is overwritten upon each calculation. The processor updates theminimum and maximum CANH/L dominant differential measurement in thedatabase, if appropriate. The maximum (MAX) and minimum (MIN) values areoverwritten if the current value is greater than or less than the valuethen stored in the data base.

[0056] The main processor calculates the Relative Node Common Voltage byadding one-half of the CANH/L differential voltage measured to the CANLvoltage (the Node Common Voltage is the mid-point between the CANH andCANL voltages for any given dominant bit). The main processor updatesthe highest and lowest Relative Node Common Voltage in the database. Thehighest and lowest values of Relative Node Common Voltage areoverwritten if the current value is greater or less than the previousvalues stored in the database respectively.

[0057] After each message frame is received, the main processor alsocalculates the network Common Mode Voltage by subtracting the lowestRelative Node Common Voltage in the database from the highest RelativeNode Common Voltage. The previously stored value is overwritten, if thenew value is greater than the previously stored value. The Common ModeVoltage measurement always reflect the worst-case or maximum differencebetween all Relative Node Common Voltages measured since the meter wasturned on or re-set by the operator.

[0058] (C) Measurements Taken Between Frame and Calculations Made

[0059] Between message frames, when the bus is in an idle condition, themain processor receives or captures CANH and CANL voltages. The bus isrecessive or idle. The data processor then copies the CANH and CANLrecessive measurements into the database. The previous “live” (i.e.,real time as contrasted with derived or computed) values areoverwritten. The main data processor then updates the minimum andmaximum CANH and CANL recessive measurements in the database, ifappropriate. The maximum and minimum values of CANH and CANL recessiveare overwritten if the current value is greater or less than theprevious values in the database respectively.

[0060] The main data processor calculates the CANH/L recessivedifferential voltage by subtracting the CANL value from the CANH value.This calculated value is then copied into the database. The previous“live” value is overwritten (i.e., stored in the same memory address asthe previous value had been stored). The main data processor thenupdates the minimum and maximum CANH/L recessive differentialmeasurements in the database, if appropriate. The maximum and minimumvalues are overwritten by the data processor if the current value isgreater or less than the previous values stored in the database.

[0061] In summary, there are a total of nineteen CAN voltagemeasurements recorded in the database of the system:

[0062] 1. “Live” CANH recessive voltage (most recent measurement);

[0063] 2. Maximum CANH recessive voltage;

[0064] 3. Minimum CANH recessive voltage;

[0065] 4. “Live” CANL recessive voltage (most recent measurement);

[0066] 5. Maximum CANL recessive voltage;

[0067] 6. Minimum CANL recessive voltage;

[0068] 7. “Live” CANH/L differential recessive voltage (most recentmeasurement);

[0069] 8. Maximum CANH/L differential recessive voltage;

[0070] 9. Minimum CANH/L differential recessive voltage;

[0071] 10. “Live” CANH dominant voltage (most recent measurement);

[0072] 11. Maximum CANH dominant voltage;

[0073] 12. Minimum CANH dominant voltage;

[0074] 13. “Live” CANL dominant voltage (most recent measurement);

[0075] 14. Maximum CANL dominant voltage;

[0076] 15. Minimum CANL dominant voltage;

[0077] 16. “Live” CANH/L differential dominant voltage (most recentmeasurement);

[0078] 17. Maximum CANH/L differential dominant voltage;

[0079] 18. Minimum CANH/L differential dominant voltage;

[0080] 19. Network Common Mode voltage.

[0081] (D) Other Measurements Taken

[0082] Using conventional software, hardware or a combination of meansthereof, the meter measures communication errors and network trafficstatistics for the entire network and for each station. As with theelectrical measurements described in detail, the meter records “live”,minimum and maximum values of these measurements.

[0083] In summary, the meter of the present invention measures andrecords, from the observation point of the meter connection and for eachnode on the network, a series of signals, including the CANH recessivevalue (displayed at Position 8 of the rotary switch 19), the CANHdominant value (Position 9), the CANL recessive value (Position 10) andthe CANL dominant value (Position 11). The minimum and maximum values ofthese signals which have been recorded and overwritten during testingsince the last reset of the meter or since the meter was turned on italso may be viewed by sequentially pressing the switch 15 (sometimesreferred to as the incrementing switch) to the present minimum, maximum,and current or “live” values in sequence, each being displayed in valueand indicated as such on the display 13. The meter of the presentinvention constantly measures and records statistics related tocommunication errors (Position 1) and network traffic (Position 2). Theminimum and maximum values of these measurements which have beenrecorded and overwritten since the last reset of the meter or since themeter was turned on may also be viewed by sequentially pressing theswitch 15 to present minimum, maximum and current or “live” values insequence. This information alone is valuable in diagnosing thereliability or acceptability of a functioning network, but it has beenfound that an even more significant factor is the calculation, from thestored values, of the Common Mode Voltage (Position 5) which is definedas a worst-case maximum between any two nodes on the network, i.e.,voltage for every node in the network as represented by the arrow 46 inFIG. 3.

[0084] If a particular measurement under investigation is found to bewithin desired operating specifications, the meter signifies a “happy”face indicated by the symbol 48 on the display 12 in FIG. 1. Similarly,if a particular measurement under investigation is not within thedesired operating specifications, but is also not within theunacceptable specifications, a “neutral” face is illuminated asindicated by reference number 49 in FIG. 1. Third, if a particularmeasurement under investigation is found to be unacceptable, the systemilluminates a “sad” face symbol indicated at 50 in FIG. 1.

[0085] Turning now to FIG. 9, there is a shown a flow chart for the“Auto Search” mode of operation which is entered when the operator turnsthe rotary switch 19 to the upper or twelve o'clock position, as seen inFIG. 1. The operational program for the “Auto Search” mode may be storedin the interface processor 30. The processor 30 selects the firstmeasurement in the priority sequence, as indicated by block 70 in FIG.9. The processor then determines, in block 71, whether that measurementis within acceptable limits or whether is it unacceptable or whether itis in the intermediate or “neutral” range and is, therefore,conditionally acceptable. If the signal value is conditionallyacceptable or unacceptable, the program proceeds to block 72 anddisplays either the conditionally acceptable symbol 49 or theunacceptable symbol 50 respectively. If, in block 71, the processor 30determines that the value is acceptable, the processor proceeds toselect the next measurement in the priority sequence as indicated inblock 74 without displaying anything. After all the indicatedmeasurements are checked in block 75, if all the values are withinacceptable range, processor displays the acceptable symbol 48, asindicated in block 76 and stops for that cycle. If, in decision block75, any one or more of the measurements is unacceptable, the programproceeds back to the beginning of block 71 to make the samedeterminations already indicated.

[0086] Having thus disclosed in detail one embodiment of the invention,persons skilled in the art will be able to modify certain of thecircuitry which has been disclosed and to substitute equivalent elementsfor those described while continuing to practice the principle of theinvention; and it is therefore intended that all such modifications andsubstitutions be covered as they are embraced within the spirit andscope of the appended claims.

1. A test meter for a data communication network having first and seconddata lines, data bits on said network being represented by signals insaid data lines including a relatively high signal on one of said linesand relatively low signal on the other of said lines and includingdominant and recessive bits occurring in succession comprising: an inputconnector for connecting to said network; a transceiver receiving dataon said data lines and generating an output stream of data signals inaccordance with a protocol of said network; a data processor receivingthe output stream of said transceiver and processing the same accordingto a stored program; at least first and second analog-to-digitalconverters having controlled sampling times and coupled respectively tosaid input connector for generating sampled digital signalrepresentations of the voltage on each of said data lines of saidnetwork respectively, said data processor coupled to receive the outputsignals of said analog-to-digital converters; a visual display having anumeric display and indicia display for representing acceptable orunacceptable conditions being measured by said meter; said dataprocessor being programmed to enter an auto search mode upon actuationby a user to cycle through predetermined network operating parametersincorporating the signals on said first and second data lines, said dataprocessor storing data representative of an acceptable state ofconditions and an unacceptable state of conditions for each networkparameter being measured, said data processor controlling the samplingof said analog-to-digital connectors and comparing the measured datafrom said analog-to-digital converts to associated data of said storeddata and actuating said indicia display to evidence one of saidacceptable and unacceptable conditions for selected ones of said networkparameters, and actuating said numeric display to indicate the valuethereof.
 2. The apparatus of claim 1 wherein said network incorporatesDeviceNet specifications having CANH signal on said first data line andCANL signal on said second data line; and characterized in said networkparameters measured and stored by said data processor include: (a) thevalues of said CANH signal for a dominant bit; (b) the value of saidCANH signal for a recessive bit; (c) the value of said CANL signal for adominant bit; and (d) the value of said CANL signal for a recessive bit.3. The apparatus of claim 2 wherein said system parameters measured andstored by said data processor further include: (e) a differential valuebetween said CANH and said CANL signals at a pre-determined samplingtime for a dominant bit; and (f) a differential value between said CANHand said CANL at a pre-determined sampling time for a recessive bit,said data processor being programmed to compute said differential valuesfrom stored measurements of said CANH and CANL signals.
 4. The apparatusof claim 3 wherein said data processor upon initial action by a usercycles sequentially through each of said measurements or calculationsfor said network parameters and actuates said display to display onlyunacceptable results, and further including a switch actuable by a userto sequence through each of said measured or calculated parameters andto continue through said sequence when said parameters are determined tobe within acceptable limits, said data processor stopping saidsequencing when a network parameter is determined to be unacceptable andactuating said indicia display to indicate an unacceptable condition. 5.The meter of claim 1 wherein said data processor further includes datarepresentative of threshold values of each of said CANH and CANL signalswhich define ranges of acceptable, unacceptable and marginal values forsaid network parameters, and wherein said indicia display includes avisual indicator conveying that an associated signal is within saidneutral range.
 6. The meter of claim 5 wherein said visual indicator ofacceptable measured levels is a smiling face.
 7. The meter of claim 6wherein said visual indicator of unacceptable measured levels is a sadface.
 8. The meter of claim 7 wherein said visual indicator of marginalmeasured levels is a neutral face.
 9. The apparatus of claim 1 whereinsaid network protocol is DeviceNet protocol and transmits binarymessages in frames having an RTR bit, and wherein said data processormeasures CANH and CANL by sampling said RTR bit in each data frame toavoid ambiguity among competing devices on said network at the start ofa message frame, and wherein each sample is taken at the normal MAC bitsampling time under DeviceNet specifications.
 10. The apparatus of claim1 further including a manual incrementing switch actuable by a user,said data processor responsive to positions of said switch to incrementsaid display to the next succeeding signal parameter, and actuating saiddisplay to identify the parameter being displayed and its value.
 11. Themeter of claim 1 wherein said network employs DeviceNet protocol andsaid network operating parameters include CANH for a dominant bit, CANHfor a recessive bit, CANL for a dominant bit and CANL for a recessivebit for each data frame and wherein said data processor stores the mostrecent value for each of said parameters; and wherein said incrementingswitch has a separate position to display each of said recorded networkparameters.
 12. The apparatus of claim 11 wherein said networkparameters further include a differential voltage value of CANH/CANL fora dominant bit and a differential voltage value of CANH/CANL for arecessive bit, said incrementing switch having a separate position foreach of said differential values.
 13. The apparatus of claim 12 whereinsaid network includes a shield connection and a bus power line coupledto said connector when said connector is connected to said network andwherein said data processor measures and stores values for bus powervoltage and shield voltage and display to displays said values when anoperator actuates said rotary switch to an associated position.
 14. Themeter of claim 13 wherein said data processor further computes andstores maximum and minimum values for said network parameters, said dataprocessor overwriting previously stored maximum and minimum values forsaid network parameters when said stored values are exceeded by currentvalues.
 15. The meter of claim 14 wherein data processor is responsiveto switch to display current or live values of said network parameter, astored maximum value and a stored minimum value upon successiveactuations of said incrementing switch.
 16. The meter of claim 1 whereinsaid data processor computes the Relative Node Common Voltage from thestored database and determines and stores the maximum difference betweenany two stored values for said Relative Node Common Voltage for saidnetwork said difference being Common Mode Voltage for said network, saiddata processor actuating said display to display said common ModeVoltage when said incrementing switch is set to a predetermined positionby a user.